Yttrium-labeled IDEC anti-CD20 murine monoclonal antibody (IDEC- Y2B8) in combination wtih the chimeric IgG1 kappa anti-CD20 monoclonal antibody Rituxan is under investigation for treatment of patients with relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL). The objectives of this trial are: 1) to determine the efficacy of IDEC- Y2B8 therapy in patients with relapsed or refractory, low grade or follicular or transformed B-cell non-Hodgkin's lymphoma whose disease did not respond (did not achieve a Pr or CR) to previous treatment with Rituxan. The primary efficacy variable will be overall response rate, secondary efficacy variables wil lbe duration of response and time to progression, and additional analyses will include complete, clinical complete and partial response rate, and time to next anti-cancer treatment and 2) to characterize (qualitative, quantitative, duration and reversibility) the safety (adverse events) of both IDEC-Y2B8 as a therapeutic agent and IDEC-In2B8 as an imaging agent when administered in conjunction with Rituxan.